Isoprene degradation in soil: evidence for competition between chemical and biological degradation in response to isoprene priming

Abstract The degradation of isoprene – a prevalent volatile organic compound (VOC) – in soil has primarily been attributed as a microbial process, but chemical degradation may also play a role. Separating simultaneous abiotic and biotic degradation pathways under representative conditions has been a technical challenge, leaving the fate of surface and subsurface isoprene inputs from the atmosphere, litter roots, and microbes to the soil uncertain. Here, we investigated the real-time dynamics of belowground isoprene degradation by introducing isoprene into the subsurface through an artificial root and tracking its fate along with primary gas-phase oxidation products via in situ soil gas probes and online high-resolution proton transfer reaction time-of-flight mass spectrometry. Isoprene additions generated oxidation products from known NO- and •OH-initiated pathways, revealing chemical degradation as an active loss pathway for isoprene in soil. Over time, isoprene concentrations plummeted relative to an inert tracer despite continuous and repeated isoprene addition, revealing a lasting up-regulation of microbial isoprene degradation that ultimately outcompeted the chemical sink. We verified the presence of putative bacterial isoprene-degrading genes in the soil by quantitative PCR of isoA and identified microbial groups that increased in abundance in response to isoprene availability using 16S amplicon sequencing. Overall, our results show for the first time the relevance of chemical degradation pathways to isoprene in soil and the capacity and dynamics of the soil microbiome to respond using community memory to increased isoprene availability. Soil is a dynamic oxidative and adaptive environment beneath our feet that may play additional roles in the biosphere–atmosphere exchange of isoprene, and by extension other VOCs, beyond what was previously expected.